source: branches/samba-3.3.x/docs-xml/Samba3-HOWTO/TOSHARG-AccessControls.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE chapter PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc">

<chapter id="AccessControls">
<chapterinfo>
        &author.jht;
        &author.jeremy;
        <author>&person.jelmer;<contrib>drawing</contrib></author>
        <pubdate>May 10, 2003</pubdate>
</chapterinfo>
<title>File, Directory, and Share Access Controls</title>

<para>
<indexterm><primary>ACLs</primary></indexterm>
<indexterm><primary>share</primary></indexterm>
<indexterm><primary>network access controls</primary></indexterm>
<indexterm><primary>unauthorized access</primary></indexterm>
Advanced MS Windows users are frequently perplexed when file, directory, and share manipulation of
resources shared via Samba do not behave in the manner they might expect. MS Windows network
administrators are often confused regarding network access controls and how to
provide users with the access they need while protecting resources from unauthorized access.
</para>

<para>
<indexterm><primary>file access permissions</primary></indexterm>
<indexterm><primary>directory access permissions</primary></indexterm>
Many UNIX administrators are unfamiliar with the MS Windows environment and in particular
have difficulty in visualizing what the MS Windows user wishes to achieve in attempts to set file
and directory access permissions. 
</para>

<para>
<indexterm><primary>bridge</primary></indexterm>
<indexterm><primary>directory controls</primary></indexterm>
<indexterm><primary>directory permissions</primary></indexterm>
<indexterm><primary></primary></indexterm>
The problem lies in the differences in how file and directory permissions and controls work
between the two environments. This difference is one that Samba cannot completely hide, even
though it does try to bridge the chasm to a degree.
</para>

<para>
<indexterm><primary>Extended Attributes</primary></indexterm>
<indexterm><primary>ACLs</primary><secondary>POSIX</secondary></indexterm>
<indexterm><primary>Access Control List</primary></indexterm>
<indexterm><primary>commercial Linux products</primary></indexterm>
POSIX Access Control List technology has been available (along with extended attributes)
for UNIX for many years, yet there is little evidence today of any significant use. This
explains to some extent the slow adoption of ACLs into commercial Linux products. MS Windows
administrators are astounded at this, given that ACLs were a foundational capability of the now
decade-old MS Windows NT operating system.
</para>

<para>
<indexterm><primary>network administrator</primary></indexterm>
The purpose of this chapter is to present each of the points of control that are possible with
Samba-3 in the hope that this will help the network administrator to find the optimum method
for delivering the best environment for MS Windows desktop users.
</para>

<para>
<indexterm><primary>interoperability</primary></indexterm>
<indexterm><primary>data interchange</primary></indexterm>
This is an opportune point to mention that Samba was created to provide a means of interoperability
and interchange of data between differing operating environments. Samba has no intent to change
UNIX/Linux into a platform like MS Windows. Instead the purpose was and is to provide a sufficient
level of exchange of data between the two environments. What is available today extends well
beyond early plans and expectations, yet the gap continues to shrink.
</para>

<sect1>
<title>Features and Benefits</title>

        <para>
        Samba offers much flexibility in file system access management. These are the key access control
        facilities present in Samba today:
        </para>

        <itemizedlist>
        <title>Samba Access Control Facilities</title>
                <listitem><para>
                <indexterm><primary>permissions</primary><secondary>UNIX file and directory</secondary></indexterm>
                <emphasis>UNIX File and Directory Permissions</emphasis>
                </para>

                        <para>
<indexterm><primary>UNIX file system access controls</primary></indexterm>
<indexterm><primary>access controls</primary></indexterm>
<indexterm><primary>permissions and controls</primary></indexterm>
                        Samba honors and implements UNIX file system access controls. Users
                        who access a Samba server will do so as a particular MS Windows user.
                        This information is passed to the Samba server as part of the logon or
                        connection setup process. Samba uses this user identity to validate
                        whether or not the user should be given access to file system resources
                        (files and directories). This chapter provides an overview for those
                        to whom the UNIX permissions and controls are a little strange or unknown.
                        </para>
                </listitem>

                <listitem><para>
                <emphasis>Samba Share Definitions</emphasis>
                </para>

                        <para>
<indexterm><primary>share settings</primary></indexterm>
                        In configuring share settings and controls in the &smb.conf; file,
                        the network administrator can exercise overrides to native file
                        system permissions and behaviors. This can be handy and convenient
                        to effect behavior that is more like what MS Windows NT users expect,
                        but it is seldom the <emphasis>best</emphasis> way to achieve this.
                        The basic options and techniques are described herein.
                        </para>
                </listitem>

                <listitem><para>
                <emphasis>Samba Share ACLs</emphasis>
                <indexterm><primary>ACLs</primary><secondary>share</secondary></indexterm>
                </para>

                        <para>
<indexterm><primary>ACLs on shares</primary></indexterm>
                        Just as it is possible in MS Windows NT to set ACLs on shares
                        themselves, so it is possible to do in Samba.
                        Few people make use of this facility, yet it remains one of the
                        easiest ways to affect access controls (restrictions) and can often
                        do so with minimum invasiveness compared with other methods.
                        </para>
                </listitem>

                <listitem><para>
                                <indexterm><primary>ACLs</primary><secondary>POSIX</secondary></indexterm>
                                <indexterm><primary>ACLs</primary><secondary>Windows</secondary></indexterm>
                <emphasis>MS Windows ACLs through UNIX POSIX ACLs</emphasis>
                </para>

                        <para>
<indexterm><primary>native ACLs</primary></indexterm>
                        The use of POSIX ACLs on UNIX/Linux is possible only if the underlying
                        operating system supports them. If not, then this option will not be
                        available to you. Current UNIX technology platforms have native support
                        for POSIX ACLs. There are patches for the Linux kernel that also provide
                        this support. Sadly, few Linux platforms ship today with native ACLs and
                        extended attributes enabled. This chapter has pertinent information
                        for users of platforms that support them.
                        </para>
                </listitem>
        </itemizedlist>

</sect1>

<sect1>
<title>File System Access Controls</title>

<para>
Perhaps the most important recognition to be made is the simple fact that MS Windows NT4/200x/XP
implement a totally divergent file system technology from what is provided in the UNIX operating system
environment. First we consider what the most significant differences are, then we look
at how Samba helps to bridge the differences.
</para>

        <sect2>
        <title>MS Windows NTFS Comparison with UNIX File Systems</title>

        <para>
        <indexterm><primary>NTFS</primary></indexterm>
        <indexterm><primary>File System</primary></indexterm>
        <indexterm><primary>File System</primary><secondary>UNIX</secondary></indexterm>
        <indexterm><primary>File System</primary><secondary>Windows</secondary></indexterm>
        Samba operates on top of the UNIX file system. This means it is subject to UNIX file system conventions
        and permissions. It also means that if the MS Windows networking environment requires file system
        behavior, that differs from UNIX file system behavior then somehow Samba is responsible for emulating
        that in a transparent and consistent manner.
        </para>

        <para>
        It is good news that Samba does this to a large extent, and on top of that, provides a high degree
        of optional configuration to override the default behavior. We look at some of these overrides,
        but for the greater part we stay within the bounds of default behavior. Those wishing to explore
        the depths of control ability should review the &smb.conf; man page.
        </para>

        <para>The following compares file system features for UNIX with those of MS Windows NT/200x:
        <indexterm><primary>File System</primary><secondary>feature comparison</secondary></indexterm>
        
        </para>

        <variablelist>
                <varlistentry>
                        <term>Name Space</term>
                        <listitem>
                <para>
                MS Windows NT4/200x/XP file names may be up to 254 characters long, and UNIX file names
                may be 1023 characters long. In MS Windows, file extensions indicate particular file types;
                in UNIX this is not so rigorously observed because all names are considered arbitrary. 
                </para>
                <para>
                What MS Windows calls a folder, UNIX calls a directory.
                </para>
                        </listitem>
                </varlistentry>

                <varlistentry>
                        <term>Case Sensitivity</term>
                        <listitem>
                <para>
                <indexterm><primary>8.3 file names</primary></indexterm>
                <indexterm><primary>File System</primary><secondary>case sensitivity</secondary></indexterm>
                MS Windows file names are generally uppercase if made up of 8.3 (8-character file name
                and 3 character extension. File names that are longer than 8.3 are case preserving and case
                insensitive.
                </para>

                <para>
                UNIX file and directory names are case sensitive and case preserving. Samba implements the
                MS Windows file name behavior, but it does so as a user application. The UNIX file system
                provides no mechanism to perform case-insensitive file name lookups. MS Windows does this
                by default. This means that Samba has to carry the processing overhead to provide features
                that are not native to the UNIX operating system environment.
                </para>

                <para>
                Consider the following. All are unique UNIX names but one single MS Windows file name:
                <screen>
                                MYFILE.TXT
                                MyFile.txt
                                myfile.txt
                </screen></para>

                <para>
                So clearly, in an MS Windows file namespace these three files cannot co-exist, but in UNIX
                they can.
                </para>

                <para>
                So what should Samba do if all three are present? That which is lexically first will be
                accessible to MS Windows users; the others are invisible and unaccessible &smbmdash; any
                other solution would be suicidal. The Windows client will ask for a case-insensitive file
                lookup, and that is the reason for which Samba must offer a consistent selection in the
                event that the UNIX directory contains multiple files that would match a case insensitive
                file listing.
                </para></listitem>
                </varlistentry>

                <varlistentry>
                <term>Directory Separators</term>
                <listitem><para>
                <indexterm><primary>Directory Separators</primary></indexterm>
                MS Windows and DOS use the backslash <constant>\</constant> as a directory delimiter, and UNIX uses
                the forward-slash <constant>/</constant> as its directory delimiter. This is handled transparently by Samba.
                </para></listitem>
                </varlistentry>

                <varlistentry>
                <term>Drive Identification</term>
                <listitem><para>
                <indexterm><primary>Drive Identification</primary></indexterm>
                MS Windows products support a notion of drive letters, like <command>C:</command>, to represent
                disk partitions. UNIX has no concept of separate identifiers for file partitions; each
                such file system is mounted to become part of the overall directory tree.
                The UNIX directory tree begins at <constant>/</constant> just as the root of a DOS drive is specified as
                <constant>C:\</constant>.
                </para></listitem>
                </varlistentry>

                <varlistentry>
                <term>File Naming Conventions</term>
                <listitem><para>
                <indexterm><primary>File Naming Conventions</primary></indexterm>
                MS Windows generally never experiences file names that begin with a dot (<constant>.</constant>), while in UNIX these
                are commonly found in a user's home directory. Files that begin with a dot (<constant>.</constant>) are typically
                startup files for various UNIX applications, or they may be files that contain
                startup configuration data.
                </para></listitem>
                </varlistentry>

                <varlistentry>
                <term>Links and Short-Cuts</term>
                <listitem><para>
                <indexterm><primary>Links</primary><secondary>hard</secondary></indexterm>
                <indexterm><primary>Links</primary><secondary>soft</secondary></indexterm>
                <indexterm><primary>Shortcuts</primary></indexterm>
                MS Windows make use of <emphasis>links and shortcuts</emphasis> that are actually special types of files that will
                redirect an attempt to execute the file to the real location of the file. UNIX knows of file and directory
                links, but they are entirely different from what MS Windows users are used to.
                </para>

                <para>
                Symbolic links are files in UNIX that contain the actual location of the data (file or directory). An
                operation (like read or write) will operate directly on the file referenced. Symbolic links are also
                referred to as <quote>soft links.</quote> A hard link is something that MS Windows is not familiar with. It allows
                one physical file to be known simultaneously by more than one file name.
                </para></listitem>
                </varlistentry>
        </variablelist>

        <para>
        There are many other subtle differences that may cause the MS Windows administrator some temporary discomfort
        in the process of becoming familiar with UNIX/Linux. These are best left for a text that is dedicated to the
        purpose of UNIX/Linux training and education.
        </para>

        </sect2>

        <sect2>
        <title>Managing Directories</title>

        <para>
<indexterm><primary>create</primary></indexterm>
<indexterm><primary>delete</primary></indexterm>
<indexterm><primary>rename</primary></indexterm>
        There are three basic operations for managing directories: <command>create</command>, <command>delete</command>,
        <command>rename</command>. <link linkend="TOSH-Accesstbl">Managing Directories with UNIX and
        Windows</link> compares the commands in Windows and UNIX that implement these operations.
        </para>

        <table frame="all" id="TOSH-Accesstbl">
                <title>Managing Directories with UNIX and Windows</title>
                <tgroup align="center" cols="3">
                <thead>
                <row><entry>Action</entry><entry>MS Windows Command</entry><entry>UNIX Command</entry></row>
                </thead>
        
                <tbody>
                        <row><entry>create</entry><entry>md folder</entry><entry>mkdir folder</entry></row>
                        <row><entry>delete</entry><entry>rd folder</entry><entry>rmdir folder</entry></row>
                        <row><entry>rename</entry><entry>rename oldname newname</entry><entry>mv oldname newname</entry></row>
                </tbody>
        </tgroup>
        </table>

        </sect2>

        <sect2>
        <title>File and Directory Access Control</title>

        <para>
        <indexterm><primary>ACLs</primary><secondary>File System</secondary></indexterm>
<indexterm><primary>POSIX ACLs</primary></indexterm>
<indexterm><primary>EAs</primary></indexterm>
        The network administrator is strongly advised to read basic UNIX training manuals and reference materials
        regarding file and directory permissions maintenance. Much can be achieved with the basic UNIX permissions
        without having to resort to more complex facilities like POSIX ACLs or extended attributes (EAs).
        </para>

        <para>
        UNIX/Linux file and directory access permissions involves setting three primary sets of data and one control set.
        A UNIX file listing looks as follows:
<screen>
&prompt;<userinput>ls -la</userinput>
total 632
drwxr-xr-x   13 maryo   gnomes      816 2003-05-12 22:56 .
drwxrwxr-x   37 maryo   gnomes     3800 2003-05-12 22:29 ..
dr-xr-xr-x    2 maryo   gnomes       48 2003-05-12 22:29 muchado02
drwxrwxrwx    2 maryo   gnomes       48 2003-05-12 22:29 muchado03
drw-rw-rw-    2 maryo   gnomes       48 2003-05-12 22:29 muchado04
d-w--w--w-    2 maryo   gnomes       48 2003-05-12 22:29 muchado05
dr--r--r--    2 maryo   gnomes       48 2003-05-12 22:29 muchado06
drwsrwsrwx    2 maryo   gnomes       48 2003-05-12 22:29 muchado08
----------    1 maryo   gnomes     1242 2003-05-12 22:31 mydata00.lst
--w--w--w-    1 maryo   gnomes     7754 2003-05-12 22:33 mydata02.lst
-r--r--r--    1 maryo   gnomes    21017 2003-05-12 22:32 mydata04.lst
-rw-rw-rw-    1 maryo   gnomes    41105 2003-05-12 22:32 mydata06.lst
&prompt;
</screen>
        </para>

        <para>
        The columns represent (from left to right) permissions, number of hard links to file, owner, group, size
        (bytes), access date, time of last modification, and file name.
        </para>

        <para>
        An overview of the permissions field is shown in <link linkend="access1">Overview of UNIX permissions
        field</link>.
        </para>

        <figure id="access1">
                <title>Overview of UNIX permissions field.</title>
                <imagefile scale="40">access1</imagefile>
        </figure>

        <para>
                Any bit flag may be unset. An unset bit flag is the equivalent of "cannot" and is represented
                as a <quote>-</quote> character (see <link linkend="access2"/>)
<indexterm><primary>read</primary></indexterm>
<indexterm><primary>write</primary></indexterm>
<indexterm><primary>execute</primary></indexterm>
<indexterm><primary>user</primary></indexterm>
<indexterm><primary>group</primary></indexterm>
<indexterm><primary>other</primary></indexterm>
        </para>

<example id="access2">
<title>Example File</title>
<programlisting>
-rwxr-x---   Means: 
 ^^^                The owner (user) can read, write, execute
    ^^^             the group can read and execute
       ^^^          everyone else cannot do anything with it.
</programlisting>
</example>


        <para>
<indexterm><primary>character device</primary></indexterm>
<indexterm><primary>block device</primary></indexterm>
<indexterm><primary>pipe device</primary></indexterm>
<indexterm><primary>UNIX Domain Socket</primary></indexterm>
        Additional possibilities in the [type] field are c = character device, b = block device, p = pipe device,
        s = UNIX Domain Socket.
        </para>

        <para>
<indexterm><primary>read</primary></indexterm>
<indexterm><primary>write</primary></indexterm>
<indexterm><primary>execute</primary></indexterm>
<indexterm><primary>SGID</primary></indexterm>
<indexterm><primary>SUID</primary></indexterm>
        The letters <constant>rwxXst</constant> set permissions for the user, group, and others as read (r), write (w),
        execute (or access for directories) (x), execute  only  if  the  file  is a directory or already has execute
        permission for some user (X), set user (SUID) or group ID (SGID) on execution (s), sticky (t).
        </para>

        <para>
<indexterm><primary>sticky bit</primary></indexterm>
<indexterm><primary>unlinked</primary></indexterm>
<indexterm><primary>/tmp</primary></indexterm>
<indexterm><primary>world-writable</primary></indexterm>
        When the sticky bit is set on a directory, files in that directory may be unlinked (deleted) or renamed only by root or their owner. 
        Without the sticky  bit, anyone able to write to the directory can delete or rename files. The sticky bit is commonly found on
        directories, such as <filename>/tmp</filename>, that are world-writable.
        </para>

        <para>
<indexterm><primary>write</primary></indexterm>
<indexterm><primary>read</primary></indexterm>
<indexterm><primary>setting up directories</primary></indexterm>
<indexterm><primary>set user id</primary><see>SUID</see></indexterm>
<indexterm><primary>set group id</primary><see>SGID</see></indexterm>
        When the set user or group ID bit (s) is set on a directory, then all files created within it will be owned by the user and/or
        group whose `set user or group' bit is set. This can be helpful in setting up directories for which it is desired that
        all users who are in a group should be able to write to and read from a file, particularly when it is undesirable for that file
        to be exclusively owned by a user whose primary group is not the group that all such users belong to.
        </para>

        <para>
        When a directory is set <constant>d-wx--x---</constant>, the owner can read and create (write) files in it, but because
        the (r) read flags are not set, files cannot be listed (seen) in the directory by anyone. The group can read files in the
        directory but cannot create new files. If files in the directory are set to be readable and writable for the group, then
        group members will be able to write to (or delete) them.
        </para>

        <sect3>
        <title>Protecting Directories and Files from Deletion</title>

        <para>
<indexterm><primary>protect files</primary></indexterm>
<indexterm><primary>protect directories</primary></indexterm>
<indexterm><primary>access controls</primary></indexterm>
<indexterm><primary>capability to delete</primary></indexterm>
        People have asked on the Samba mailing list how is it possible to protect files or directories from deletion by users.
        For example, Windows NT/2K/XP provides the capacity to set access controls on a directory into which people can
        write files but not delete them. It is possible to set an ACL on a Windows file that permits the file to be written to
        but not deleted. Such concepts are foreign to the UNIX operating system file space. Within the UNIX file system
        anyone who has the ability to create a file can write to it. Anyone who has write permission on the
        directory that contains a file and has write permission for it has the capability to delete it.
        </para>

        <para>
<indexterm><primary>directory permissions</primary></indexterm>
<indexterm><primary>delete a file</primary></indexterm>
<indexterm><primary>write access</primary></indexterm>
        For the record, in the UNIX environment the ability to delete a file is controlled by the permissions on
        the directory that the file is in. In other words, a user can delete a file in a directory to which that
        user has write access, even if that user does not own the file.
        </para>
        
        <para>
<indexterm><primary>file system capabilities</primary></indexterm>
<indexterm><primary>inheritance</primary></indexterm>
<indexterm><primary>POSIX ACLs</primary></indexterm>
<indexterm><primary>extended attributes</primary></indexterm>
        Of necessity, Samba is subject to the file system semantics of the host operating system. Samba is therefore
        limited in the file system capabilities that can be made available through Windows ACLs, and therefore performs
        a "best fit" translation to POSIX ACLs. Some UNIX file systems do, however support, a feature known
        as extended attributes. Only the Windows concept of <emphasis>inheritance</emphasis> is implemented by Samba through
        the appropriate extended attribute.
        </para> 

        <para>
<indexterm><primary>extended attributes</primary></indexterm>
<indexterm><primary>immutible</primary></indexterm>
<indexterm><primary>chattr</primary></indexterm>
<indexterm><primary>CAP_LINUX_IMMUTABLE</primary></indexterm>
        The specific semantics of the extended attributes are not consistent across UNIX and UNIX-like systems such as Linux.
        For example, it is possible on some implementations of the extended attributes to set a flag that prevents the directory
        or file from being deleted. The extended attribute that may achieve this is called the <constant>immutible</constant> bit.
        Unfortunately, the implementation of the immutible flag is NOT consistent with published documentation. For example, the
        man page for the <command>chattr</command> on SUSE Linux 9.2 says:
<screen>
A file with the i attribute cannot be modified: it cannot be deleted
or renamed, no link can be created to this file and no data can be
written to the file. Only the superuser or a process possessing the
CAP_LINUX_IMMUTABLE capability can set or clear this attribute.
</screen>
        A simple test can be done to check if the immutible flag is supported on files in the file system of the Samba host
        server.
        </para>

        <procedure>
        <title>Test for File Immutibility Support</title>

        <step><para>
        Create a file called <filename>filename</filename>.
        </para></step>

        <step><para>
        Login as the <constant>root</constant> user, then set the immutibile flag on a test file as follows:
<screen>
&rootprompt; chattr +i `filename'
</screen>
        </para></step>

        <step><para>
        Login as the user who owns the file (not root) and attempt to remove the file as follows:
<screen>
mystic:/home/hannibal > rm filename
</screen>
        It will not be possible to delete the file if the immutible flag is correctly honored.
        </para></step>
        </procedure>

        <para>
        On operating systems and file system types that support the immutible bit, it is possible to create directories
        that cannot be deleted. Check the man page on your particular host system to determine whether or not
        immutable directories are writable. If they are not, then the entire directory and its contents will effectively
        be protected from writing (file creation also) and deletion.
        </para>

        </sect3>

        </sect2>

</sect1>

<sect1>
<title>Share Definition Access Controls</title>


        <para>
        <indexterm><primary>permissions</primary><secondary>share</secondary></indexterm>
        The following parameters in the &smb.conf; file sections define a share control or affect access controls.
        Before using any of the following options, please refer to the man page for &smb.conf;.
        </para>

        <sect2>
        <title>User- and Group-Based Controls</title>

        <para>
        User- and group-based controls can prove quite useful. In some situations it is distinctly desirable to
        force all file system operations as if a single user were doing so. The use of the
        <smbconfoption name="force user"/> and <smbconfoption name="force group"/> behavior will achieve this.
        In other situations it may be necessary to use a paranoia level of control to ensure that only particular
        authorized persons will be able to access a share or its contents. Here the use of the
        <smbconfoption name="valid users"/> or the <smbconfoption name="invalid users"/> parameter may be useful.
        </para>

        <para>
        As always, it is highly advisable to use the easiest to maintain and the least ambiguous method for
        controlling access. Remember, when you leave the scene, someone else will need to provide assistance, and
        if he or she finds too great a mess or does not understand what you have done, there is risk of
        Samba being removed and an alternative solution being adopted.
        </para>

        <para>
        <link linkend="ugbc">User and Group Based Controls</link> enumerates these controls.
        </para>

        <table frame='all' pgwide='0' id="ugbc"><title>User- and Group-Based Controls</title>
        <tgroup cols='2'>
                <colspec align="left"/>
                <colspec align="justify" colwidth="1*"/>
                <thead>
                <row>
                        <entry align="center">Control Parameter</entry>
                        <entry align="center">Description, Action, Notes</entry>
                </row>
                </thead>
                <tbody>
                <row>
                        <entry><smbconfoption name="admin users"/></entry>
                        <entry><para>
                        List of users who will be granted administrative privileges on the share.
                        They will do all file operations as the superuser (root). 
                        Users in this list will be able to do anything they like on the share,
                        irrespective of file permissions.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="force group"/></entry>
                        <entry><para>
                        Specifies a UNIX group name that will be assigned as the default primary group
                        for all users connecting to this service.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="force user"/></entry>
                        <entry><para>
                        Specifies a UNIX username that will be assigned as the default user for all users connecting to this service.
                        This is useful for sharing files. Incorrect use can cause security problems.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="guest ok"/></entry>
                        <entry><para>
                        If this parameter is set for a service, then no password is required to connect to the service. Privileges will be 
                        those of the  guest account.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="invalid users"/></entry>
                        <entry><para>
                        List of users that should not be allowed to login to this service.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="only user"/></entry>
                        <entry><para>
                        Controls whether connections with usernames not in the user list will be allowed.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="read list"/></entry>
                        <entry><para>
                        List of users that are given read-only access to a service. Users in this list
                        will not be given write access, no matter what the  read-only  option is set to. 
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="username"/></entry>
                        <entry><para>
                        Refer to the &smb.conf; man page for more information; this is a complex and potentially misused parameter.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="valid users"/></entry>
                        <entry><para>
                        List of users that should be allowed to login to this service.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="write list"/></entry>
                        <entry><para>
                        List of users that are given read-write access to a service.
                        </para></entry>
                </row>
                </tbody>
        </tgroup>
        </table>

        </sect2>

        <sect2>
        <title>File and Directory Permissions-Based Controls</title>

        <para>
        Directory permission-based controls, if misused, can result in considerable difficulty in diagnosing the causes of 
        misconfiguration. Use them sparingly and carefully. By gradually introducing each, one at a time, undesirable side 
        effects may be detected. In the event of a problem, always comment all of them out and then gradually reintroduce 
        them in a controlled way.
        </para>

        <para>
        Refer to <link linkend="fdpbc">File and Directory Permission Based Controls</link> for information 
        regarding the parameters that may be used to set file and directory permission-based access controls.
        </para>

        <table frame='all' id="fdpbc"><title>File and Directory Permission-Based Controls</title>
                <tgroup cols='2'>
                        <colspec align="left"/>
                        <colspec align="justify" colwidth="1*"/>
                <thead>
                <row>
                        <entry align="center">Control Parameter</entry>
                        <entry align="center">Description, Action, Notes</entry>
                </row>
                </thead>
                <tbody>
                <row>
                        <entry><smbconfoption name="create mask"/></entry>
                        <entry><para>
                        Refer to the &smb.conf; man page.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="directory mask"/></entry>
                        <entry><para>
                        The octal modes used when converting DOS modes to UNIX modes when creating UNIX directories.
                        See also directory security mask.
                        </para></entry></row>
                <row>
                        <entry><smbconfoption name="dos filemode"/></entry>
                        <entry><para>
                        Enabling this parameter allows a user who has write access to the file to modify the permissions on it.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="force create mode"/></entry>
                        <entry><para>
                        This parameter specifies a set of UNIX-mode bit permissions that will always be set on a file created by Samba.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="force directory mode"/></entry>
                        <entry><para>
                        This parameter specifies a set of UNIX-mode bit permissions that will always be set on a directory created by Samba.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="force directory security mode"/></entry>
                        <entry><para>
                        Controls UNIX permission bits modified when a Windows NT client is manipulating UNIX permissions on a directory.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="force security mode"/></entry>
                        <entry><para>
                        Controls UNIX permission bits modified when a Windows NT client manipulates UNIX permissions.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="hide unreadable"/></entry>
                        <entry><para>
                        Prevents clients from seeing the existence of files that cannot be read.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="hide unwriteable files"/></entry>
                        <entry><para>
                        Prevents clients from seeing the existence of files that cannot be written to. Unwritable directories are shown as usual. 
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="nt acl support"/></entry>
                        <entry><para>
                        This parameter controls whether smbd will attempt to map UNIX permissions into Windows NT ACLs.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="security mask"/></entry>
                        <entry><para>
                        Controls UNIX permission bits modified when a Windows NT client is manipulating the UNIX permissions on a file.
                        </para></entry>
                </row>
                </tbody>
        </tgroup>
        </table>

        </sect2>

        <sect2>
        <title>Miscellaneous Controls</title>

        <para>
        The parameters documented in <link linkend="mcoc">Other Controls</link> are often used by administrators
        in ways that create inadvertent barriers to file access. Such are the consequences of not understanding the 
        full implications of &smb.conf; file settings.
        </para>

        <table frame='all' id="mcoc"><title>Other Controls</title>
        <tgroup cols='2'>
                <colspec align="justify" colwidth="1*"/>
                <colspec align="justify" colwidth="1*"/>
                <thead>
                <row>
                        <entry align="center">Control Parameter</entry>
                        <entry align="center">Description, Action, Notes</entry>
                </row>
                </thead>
                <tbody>
                <row>
                        <entry>
                        <smbconfoption name="case sensitive"/>,
                        <smbconfoption name="default case"/>,
                        <smbconfoption name="short preserve case"/>
                        </entry>
                        <entry><para>
                        This means that all file name lookup will be done in a case-sensitive manner. 
                        Files will be created with the precise file name Samba received from the MS Windows client.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="csc policy"/></entry>
                        <entry><para>
                        Client-side caching policy parallels MS Windows client-side file caching capabilities.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="dont descend"/></entry>
                        <entry><para>
                        Allows specifying a comma-delimited list of directories that the server should always show as empty.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="dos filetime resolution"/></entry>
                        <entry><para>
                        This option is mainly used as a compatibility option for Visual C++ when used against Samba shares.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="dos filetimes"/></entry>
                        <entry><para>
                        DOS and Windows allow users to change file timestamps if they can write to the file. POSIX semantics prevent this.
                        This option allows DOS and Windows behavior.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="fake oplocks"/></entry>
                        <entry><para>
                        Oplocks are the way that SMB clients get permission from a server to locally cache file operations. If a server grants an
                        oplock, the client is free to assume that it is the only one accessing the file, and it will aggressively cache file data.
                        </para></entry>
                </row>
                <row>
                        <entry>
                        <smbconfoption name="hide dot files"/>,
                        <smbconfoption name="hide files"/>,
                        <smbconfoption name="veto files"/>
                        </entry>
                        <entry><para>
                        Note: MS Windows Explorer allows override of files marked as hidden so they will still be visible.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="read only"/></entry>
                        <entry><para>
                        If this parameter is yes, then users of a service may not create or modify files in the service's directory.
                        </para></entry>
                </row>
                <row>
                        <entry><smbconfoption name="veto files"/></entry>
                        <entry><para>
                        List of files and directories that are neither visible nor accessible.
                        </para></entry>
                </row>
                </tbody>
        </tgroup>
        </table>

        </sect2>

</sect1>

<sect1>
<title>Access Controls on Shares</title>


        <para>
<indexterm><primary>per-share access control</primary></indexterm>
<indexterm><primary>Everyone - Full Control</primary></indexterm>
<indexterm><primary>specific restrictions</primary></indexterm>
<indexterm><primary>share access</primary></indexterm>
        <indexterm><primary>permissions</primary><secondary>share ACLs</secondary></indexterm>
        This section deals with how to configure Samba per-share access control restrictions.
        By default, Samba sets no restrictions on the share itself. Restrictions on the share itself
        can be set on MS Windows NT4/200x/XP shares. This can be an effective way to limit who can
        connect to a share. In the absence of specific restrictions, the default setting is to allow
        the global user <constant>Everyone - Full Control</constant> (full control, change and read).
        </para>

        <para>
<indexterm><primary>access control</primary></indexterm>
<indexterm><primary>MMC</primary></indexterm>
<indexterm><primary>Computer Management</primary></indexterm>
        At this time Samba does not provide a tool for configuring access control settings on the share
        itself the only way to create those settings is to use either the NT4 Server Manager or the Windows 200x
        Microsoft Management Console (MMC) for Computer Management. There are currently no plans to provide
        this capability in the Samba command-line tool set.
        </para>

        <para>
<indexterm><primary>share_info.tdb</primary></indexterm>
<indexterm><primary>/usr/local/samba/var</primary></indexterm>
<indexterm><primary>tdbdump</primary></indexterm>
<indexterm><primary>tdb files</primary></indexterm>
        Samba stores the per-share access control settings in a file called <filename>share_info.tdb</filename>.
        The location of this file on your system will depend on how Samba was compiled. The default location
        for Samba's tdb files is under <filename>/usr/local/samba/var</filename>. If the <filename>tdbdump</filename>
        utility has been compiled and installed on your system, then you can examine the contents of this file
        by executing <command>tdbdump share_info.tdb</command> in the directory containing the tdb files.
        </para>

        <sect2>
        <title>Share Permissions Management</title>

                <para>
                The best tool for share permissions management is platform-dependent. Choose the best tool for your environment.
                </para>

                        <sect3>
                        <title>Windows NT4 Workstation/Server</title>
                        <para>
<indexterm><primary>manage share permissions</primary></indexterm>
<indexterm><primary>share permissions</primary></indexterm>
<indexterm><primary>NT Server Manager</primary></indexterm>
<indexterm><primary>Windows NT4</primary></indexterm>
                        The tool you need to manage share permissions on a Samba server from a Windows NT4 Workstation or Server
                        is the NT Server Manager.  Server Manager is shipped with Windows NT4 Server products but not with Windows
                        NT4 Workstation.  You can obtain the NT Server Manager for MS Windows NT4 Workstation from the Microsoft
                        web site <ulink url="http://support.microsoft.com/default.aspx?scid=kb;en-us;173673">support</ulink> section.
                        </para>

                        <procedure>
                        <title>Instructions</title>

                        <step><para>
                        Launch the <application>NT4 Server Manager</application> and click on the Samba server you want to
                        administer. From the menu select <guimenu>Computer</guimenu>, then click on
                        <guimenuitem>Shared Directories</guimenuitem>.
                        </para></step>

                        <step><para>
                        Click on the share that you wish to manage and click the <guilabel>Properties</guilabel> tab, then click
                        the <guilabel>Permissions</guilabel> tab. Now you can add or change access control settings as you wish.
                        </para></step>
                        </procedure>

                        </sect3>

                        <sect3>
                        <title>Windows 200x/XP</title>

                        <para>
<indexterm><primary>Windows NT4/200x/XP</primary></indexterm>
<indexterm><primary>ACLs on share</primary></indexterm>
<indexterm><primary>Sharing</primary></indexterm>
<indexterm><primary>Permissions</primary></indexterm>
                        On <application>MS Windows NT4/200x/XP</application> systems, ACLs on the share itself are set using
                        tools like the MS Explorer. For example, in Windows 200x, right-click on the shared folder,
                        then select <guimenuitem>Sharing</guimenuitem>, then click on <guilabel>Permissions</guilabel>. The default 
                        Windows NT4/200x permissions allow the group "Everyone" full control on the share.
                        </para>

                        <para>
<indexterm><primary>Computer Management</primary></indexterm>
<indexterm><primary>MMC</primary></indexterm>
<indexterm><primary>tool</primary></indexterm>
                        MS Windows 200x and later versions come with a tool called the <application>Computer Management</application>
                        snap-in for the MMC. This tool can be accessed via <guimenu>Control Panel ->
                        Administrative Tools -> Computer Management</guimenu>.
                        </para>

                        <procedure>
                        <title>Instructions</title>
                        <step><para>
                        After launching the MMC with the Computer Management snap-in, click the menu item <guimenuitem>Action</guimenuitem>
                        and select <guilabel>Connect to another computer</guilabel>. If you are not logged onto a domain you will be prompted
                        to enter a domain login user identifier and a password. This will authenticate you to the domain.
                        If you are already logged in with administrative privilege, this step is not offered.
                        </para></step>

                        <step><para>
                        If the Samba server is not shown in the <guilabel>Select Computer</guilabel> box, type in the name of the target
                        Samba server in the field <guilabel>Name:</guilabel>. Now click the on <guibutton>[+]</guibutton> next to 
                        <guilabel>System Tools</guilabel>, then on the <guibutton>[+]</guibutton> next to
                        <guilabel>Shared Folders</guilabel> in the left panel.
                        </para></step>

                        <step><para>
<indexterm><primary>Share Permissions</primary></indexterm>
                        In the right panel, double-click on the share on which you wish to set access control permissions.
                        Then click the tab <guilabel>Share Permissions</guilabel>. It is now possible to add access control entities
                        to the shared folder. Remember to set what type of access (full control, change, read) you
                        wish to assign for each entry.
                        </para></step>
                        </procedure>

                        <warning>
                        <para>
                        Be careful. If you take away all permissions from the <constant>Everyone</constant> user without removing
                        this user, effectively no user will be able to access the share. This is a result of what is known as
                        ACL precedence. Everyone with <emphasis>no access</emphasis> means that <constant>MaryK</constant> who is
                        part of the group <constant>Everyone</constant> will have no access even if she is given explicit full
                        control access.
                        </para>
                        </warning>

                        </sect3>
                </sect2>

</sect1>

<sect1>
<title>MS Windows Access Control Lists and UNIX Interoperability</title>

        <sect2>
                <title>Managing UNIX Permissions Using NT Security Dialogs</title>


                <para>
                <indexterm><primary>permissions</primary><secondary>file/directory ACLs</secondary></indexterm>
                Windows NT clients can use their native security settings dialog box to view and modify the
                underlying UNIX permissions.
                </para>

                <para>
                This ability is careful not to compromise the security of the UNIX host on which Samba is running and 
                still obeys all the file permission rules that a Samba administrator can set.
                </para>

                <para>
                Samba does not attempt to go beyond POSIX ACLs, so the various finer-grained access control
                options provided in Windows are actually ignored.
                </para> 

                <note>
                <para>
                All access to UNIX/Linux system files via Samba is controlled by the operating system file access controls.
                When trying to figure out file access problems, it is vitally important to find the identity of the Windows
                user as it is presented by Samba at the point of file access. This can best be determined from the
                Samba log files.
                </para>
                </note>
        </sect2>

        <sect2>
                <title>Viewing File Security on a Samba Share</title>

                <para>
                From an NT4/2000/XP client, right-click on any file or directory in a Samba-mounted drive letter
                or UNC path. When the menu pops up, click on the <guilabel>Properties</guilabel> entry at the bottom
                of the menu. This brings up the file <constant>Properties</constant> dialog box. Click on the 
                <guilabel>Security</guilabel> tab and you will see three buttons: <guibutton>Permissions</guibutton>,
                <guibutton>Auditing</guibutton>, and <guibutton>Ownership</guibutton>. The <guibutton>Auditing</guibutton>
                button will cause either an error message <errorname>"A requested privilege is not held by the client"</errorname>
                to appear if the user is not the NT administrator, or a dialog intended to allow an administrator
                to add auditing requirements to a file if the user is logged on as the NT administrator. This dialog is
                nonfunctional with a Samba share at this time, because the only useful button, the <guibutton>Add</guibutton>
                button, will not currently allow a list of users to be seen.
                </para>

        </sect2>

        <sect2>
                <title>Viewing File Ownership</title>

                <para>
                Clicking on the <guibutton>Ownership</guibutton> button brings up a dialog box telling you who owns
                the given file. The owner name will be displayed like this:
                <screen>
                <constant>SERVER\user (Long name)</constant>
                </screen>
                <replaceable>SERVER</replaceable> is the NetBIOS name of the Samba server, <replaceable>user</replaceable>
                is the username of the UNIX user who owns the file, and <replaceable>(Long name)</replaceable> is the
                descriptive string identifying the user (normally found in the GECOS field of the UNIX password database).
                Click on the <guibutton>Close</guibutton> button to remove this dialog.
                </para>

                <para>
                If the parameter <smbconfoption name="nt acl support"/> is set to <constant>false</constant>,
                the file owner will be shown as the NT user <emphasis>Everyone</emphasis>.
                </para>

                <para>
<indexterm><primary>Take Ownership</primary></indexterm>
                The <guibutton>Take Ownership</guibutton> button will not allow you to change the ownership of this file to
                yourself (clicking it will display a dialog box complaining that the user as whom you are currently logged onto
                the NT client cannot be found). The reason for this is that changing the ownership of a file is a privileged
                operation in UNIX, available only to the <emphasis>root</emphasis> user. Because clicking on this button causes
                NT to attempt to change the ownership of a file to the current user logged into the NT client, this will
                not work with Samba at this time.
                </para>

                <para>
<indexterm><primary>chown</primary></indexterm>
<indexterm><primary>ownership</primary></indexterm>
<indexterm><primary>Seclib</primary></indexterm>
                There is an NT <command>chown</command> command that will work with Samba and allow a user with administrator
                privilege connected to a Samba server as root to change the ownership of files on both a local NTFS file system
                or remote mounted NTFS or Samba drive. This is available as part of the <application>Seclib</application> NT
                security library written by Jeremy Allison of the Samba Team and is downloadable from the main Samba FTP site.
                </para>

        </sect2>

        <sect2>
                <title>Viewing File or Directory Permissions</title>

                <para>
                The third button is the <guibutton>Permissions</guibutton> button. Clicking on it brings up a dialog box
                that shows both the permissions and the UNIX owner of the file or directory. The owner is displayed like this:
                </para>

                <para><command><replaceable>SERVER</replaceable>\
                                <replaceable>user</replaceable> 
                                <replaceable>(Long name)</replaceable></command></para>

                <para><replaceable>SERVER</replaceable> is the NetBIOS name of the Samba server,
                <replaceable>user</replaceable> is the username of the UNIX user who owns the file, and
                <replaceable>(Long name)</replaceable> is the descriptive string identifying the user (normally found in the
                GECOS field of the UNIX password database).</para>

                <para>
                If the parameter <smbconfoption name="nt acl support"/> is set to <constant>false</constant>,
                the file owner will be shown as the NT user <constant>Everyone</constant>, and the permissions will be
                shown as NT <emphasis>Full Control</emphasis>.
                </para>


                <para>
                The permissions field is displayed differently for files and directories. Both are discussed next.
                </para>

                <sect3>
                <title>File Permissions</title>

                <para>
                The standard UNIX user/group/world triplet and the corresponding <constant>read, write,
                execute</constant> permissions triplets are mapped by Samba into a three-element NT ACL with the
                <quote>r</quote>, <quote>w</quote>, and <quote>x</quote> bits mapped into the corresponding NT
                permissions. The UNIX world permissions are mapped into the global NT group <constant>Everyone</constant>, followed 
                by the list of permissions allowed for the UNIX world. The UNIX owner and group permissions are displayed as an NT 
                <guiicon>user</guiicon> icon and an NT <guiicon>local group</guiicon> icon, respectively, followed by the list 
                of permissions allowed for the UNIX user and group.
                </para>

                <para>
                Because many UNIX permission sets do not map into common NT names such as <constant>read</constant>,
                <constant>change</constant>, or <constant>full control</constant>, usually the permissions will be prefixed
                by the words <constant>Special Access</constant> in the NT display list.
                </para>

                <para>
                But what happens if the file has no permissions allowed for a particular UNIX user group or world component?
                In order to  allow <emphasis>no permissions</emphasis> to be seen and modified, Samba then overloads the NT
                <constant>Take Ownership</constant> ACL attribute (which has no meaning in UNIX) and reports a component with
                no permissions as having the NT <command>O</command> bit set.  This was chosen, of course, to make it look
                like a zero, meaning zero permissions. More details on the decision behind this action are given below.
                </para>

                </sect3>
                
                <sect3>
                <title>Directory Permissions</title>

                <para>
                Directories on an NT NTFS file system have two different sets of permissions. The first set is the ACL set on the
                directory itself, which is usually displayed in the first set of parentheses in the normal <constant>RW</constant> 
                NT style. This first set of permissions is created by Samba in exactly the same way as normal file permissions are, described 
                above, and is displayed in the same way.
                </para>

                <para>
                The second set of directory permissions has no real meaning in the UNIX permissions world and represents the <constant>
                inherited</constant> permissions that any file created within this directory would inherit.
                </para>

                <para>
                Samba synthesizes these inherited permissions for NT by returning as an NT ACL the UNIX permission mode that a new file 
                created by Samba on this share would receive.
                </para>

                </sect3>

        </sect2>
                
        <sect2>
        <title>Modifying File or Directory Permissions</title>

        <para>
        Modifying file and directory permissions is as simple as changing the displayed permissions in the dialog box
        and clicking on <guibutton>OK</guibutton>. However, there are limitations that a user needs to be aware of,
        and also interactions with the standard Samba permission masks and mapping of DOS attributes that also need to
        be taken into account.
        </para>

        <para>
        If the parameter <smbconfoption name="nt acl support"/> is set to <constant>false</constant>, any attempt to
        set security permissions will fail with an <errorname>"Access Denied" </errorname> message.
        </para>

        <para>
        The first thing to note is that the <guibutton>Add</guibutton> button will not return a list of users in Samba
        (it will give an error message saying <errorname>"The remote procedure call failed and did not
        execute"</errorname>). This means that you can only manipulate the current user/group/world permissions listed
        in the dialog box. This actually works quite well because these are the only permissions that UNIX actually
        has.
        </para>

        <para>
        If a permission triplet (either user, group, or world) is removed from the list of permissions in the NT
        dialog box, then when the <guibutton>OK</guibutton> button is pressed, it will be applied as <emphasis>no
        permissions</emphasis> on the UNIX side. If you view the permissions again, the <emphasis>no
        permissions</emphasis> entry will appear as the NT <command>O</command> flag, as described above. This allows
        you to add permissions back to a file or directory once you have removed them from a triplet component.
        </para>

        <para>
        Because UNIX supports only the <quote>r</quote>, <quote>w</quote>, and <quote>x</quote> bits of an NT ACL, if
        other NT security attributes such as <constant>Delete Access</constant> are selected, they will be ignored
        when applied on the Samba server.
        </para>

        <para>
        When setting permissions on a directory, the second set of permissions (in the second set of parentheses) is
        by default applied to all files within that directory. If this is not what you want, you must uncheck the
        <guilabel>Replace permissions on existing files</guilabel> checkbox in the NT dialog before clicking on
        <guibutton>OK</guibutton>.
        </para>

        <para>
        If you wish to remove all permissions from a user/group/world  component, you may either highlight the
        component and click on the <guibutton>Remove</guibutton> button or set the component to only have the special
        <constant>Take Ownership</constant> permission (displayed as <command>O</command>) highlighted.
        </para>

        </sect2>

<?latex \newpage ?>
        <sect2>
        <title>Interaction with the Standard Samba <quote>create mask</quote> Parameters</title>

        <para>There are four parameters that control interaction with the standard Samba <parameter>create mask</parameter> parameters:
        

        <itemizedlist>
                <listitem><para><smbconfoption name="security mask"/></para></listitem>
                <listitem><para><smbconfoption name="force security mode"/></para></listitem>
                <listitem><para><smbconfoption name="directory security mask"/></para></listitem>
                <listitem><para><smbconfoption name="force directory security mode"/></para></listitem>
        </itemizedlist>

        </para>

        <para>
        When a user clicks on <guibutton>OK</guibutton> to apply the 
        permissions, Samba maps the given permissions into a user/group/world 
        r/w/x triplet set, and then checks the changed permissions for a 
        file against the bits set in the  
        <smbconfoption name="security mask"/> parameter. Any bits that 
        were changed that are not set to <emphasis>1</emphasis> in this parameter are left alone 
        in the file permissions.</para>

        <para>
        Essentially, zero bits in the <smbconfoption name="security mask"/>
        may be treated as a set of bits the user is <emphasis>not</emphasis> 
        allowed to change, and one bits are those the user is allowed to change.
        </para>

        <para>
        If not explicitly set, this parameter defaults to the same value as 
        the <smbconfoption name="create mask"/> parameter. To allow a user to modify all the
        user/group/world permissions on a file, set this parameter to 0777.
        </para>

        <para>
        Next Samba checks the changed permissions for a file against the bits set in the 
        <smbconfoption name="force security mode"/> parameter. Any bits 
        that were changed that correspond to bits set to <emphasis>1</emphasis> in this parameter 
        are forced to be set.</para>

        <para>
        Essentially, bits set in the <parameter>force security mode</parameter> parameter
        may be treated as a set of bits that, when modifying security on a file, the user 
        has always set to be <emphasis>on</emphasis>.</para>

        <para>
        If not explicitly set, this parameter defaults to the same value 
        as the <smbconfoption name="force create mode"/> parameter.
        To allow a user to modify all the user/group/world permissions on a file
        with no restrictions, set this parameter to 000. The
        <smbconfoption name="security mask"/> and <parameter>force 
        security mode</parameter> parameters are applied to the change 
        request in that order.</para>

        <para>
        For a directory, Samba performs the same operations as 
        described above for a file except it uses the parameter <parameter>
        directory security mask</parameter> instead of <parameter>security 
        mask</parameter>, and <parameter>force directory security mode
        </parameter> parameter instead of <parameter>force security mode
        </parameter>.</para>

        <para>
        The <smbconfoption name="directory security mask"/> parameter 
        by default is set to the same value as the <parameter>directory mask
        </parameter> parameter and the <parameter>force directory security 
        mode</parameter> parameter by default is set to the same value as 
        the <smbconfoption name="force directory mode"/> parameter.
        In this way Samba enforces the permission restrictions that 
        an administrator can set on a Samba share, while still allowing users 
        to modify the permission bits within that restriction.</para>

        <para>
        If you want to set up a share that allows users full control
        in modifying the permission bits on their files and directories and
        does not force any particular bits to be set <emphasis>on</emphasis>,
        then set the following parameters in the &smb.conf; file in that
        share-specific section:
        </para>

<?latex \newpage ?>
        <smbconfblock>
                <smbconfoption name="security mask">0777</smbconfoption>
                <smbconfoption name="force security mode">0</smbconfoption>
                <smbconfoption name="directory security mask">0777</smbconfoption>
                <smbconfoption name="force directory security mode">0</smbconfoption>
        </smbconfblock>

</sect2>

<sect2>
        <title>Interaction with the Standard Samba File Attribute Mapping</title>

        <note>
        <para>
        Samba maps some of the DOS attribute bits (such as <quote>read-only</quote>)
        into the UNIX permissions of a file. This means there can 
        be a conflict between the permission bits set via the security 
        dialog and the permission bits set by the file attribute mapping.
        </para>
        </note>

        <para>
        If a file has no UNIX read access for the owner, it will show up
        as <quote>read-only</quote> in the standard file attributes tabbed dialog.
        Unfortunately, this dialog is the same one that contains the security information
        in another tab.
        </para>

        <para>
        What this can mean is that if the owner changes the permissions
        to allow himself or herself read access using the security dialog, clicks on
        <guibutton>OK</guibutton> to get back to the standard attributes tab 
        dialog, and clicks on <guibutton>OK</guibutton> on that dialog, then 
        NT will set the file permissions back to read-only (as that is what 
        the attributes still say in the dialog). This means that after setting 
        permissions and clicking on <guibutton>OK</guibutton> to get back to the 
        attributes dialog, you should always press <guibutton>Cancel</guibutton> 
        rather than <guibutton>OK</guibutton> to ensure that your changes 
        are not overridden.
        </para>

        </sect2>

        <sect2>
        <title>Windows NT/200X ACLs and POSIX ACLs Limitations</title>

        <para>
        Windows administrators are familiar with simple ACL controls, and they typically
        consider that UNIX user/group/other (ugo) permissions are inadequate and not
        sufficiently fine-grained.
        </para>

        <para>
        Competing SMB implementations differ in how they handle Windows ACLs. Samba handles
        Windows ACLs from the perspective of UNIX file system administration and thus adopts
        the limitations of POSIX ACLs. Therefore, where POSIX ACLs lack a capability of the
        Windows NT/200X ACLs, the POSIX semantics and limitations are imposed on the Windows
        administrator.
        </para>

        <para>
        POSIX ACLs present an interesting challenge to the UNIX administrator and therefore
        force a compromise to be applied to Windows ACLs administration. POSIX ACLs are not
        covered by an official standard; rather, the latest standard is a draft standard
        1003.1e revision 17. This is the POSIX document on which the Samba implementation has
        been implemented.
        </para>

        <para>
        UNIX vendors differ in the manner in which POSIX ACLs are implemented. There are a
        number of Linux file systems that support ACLs. Samba has to provide a way to make
        transparent all the differences between the various implementations of POSIX ACLs.
        The pressure for ACLs support in Samba has noticeably increased the pressure to
        standardize ACLs support in the UNIX world.
        </para>

        <para>
        Samba has to deal with the complicated matter of handling the challenge of the Windows
        ACL that implements <emphasis>inheritance</emphasis>, a concept not anticipated by POSIX
        ACLs as implemented in UNIX file systems. Samba provides support for <emphasis>masks</emphasis>
        that permit normal ugo and ACLs functionality to be overrided. This further complicates
        the way in which Windows ACLs must be implemented.
        </para>

        <sect3>
        <title>UNIX POSIX ACL Overview</title>

        <para>
        In examining POSIX ACLs we must consider the manner in which they operate for 
        both files and directories. File ACLs have the following significance:
<screen>
# file: testfile      &lt;- the file name
# owner: jeremy       &lt;-- the file owner
# group: users        &lt;-- the POSIX group owner
user::rwx             &lt;-- perms for the file owner (user)
user:tpot:r-x         &lt;-- perms for the additional user `tpot'
group::r--            &lt;-- perms for the file group owner (group)
group:engrs:r--       &lt;-- perms for the additonal group `engineers'
mask:rwx              &lt;-- the mask that is `ANDed' with groups
other::---            &lt;-- perms applied to everyone else (other)
</screen>
        Directory ACLs have the following signficance:
<screen>
# file: testdir       &lt;-- the directory name
# owner: jeremy       &lt;-- the directory owner
# group: jeremy       &lt;-- the POSIX group owner
user::rwx             &lt;-- directory perms for owner (user)
group::rwx            &lt;-- directory perms for owning group (group)
mask::rwx             &lt;-- the mask that is `ANDed' with group perms
other:r-x             &lt;-- perms applied to everyone else (other)
default:user::rwx     &lt;-- inherited owner perms
default:user:tpot:rwx &lt;-- inherited extra perms for user `tpot'
default:group::r-x    &lt;-- inherited group perms
default:mask:rwx      &lt;-- inherited default mask
default:other:---     &lt;-- inherited permissions for everyone (other)
</screen>
        </para>

        </sect3>

        <sect3>
        <title>Mapping of Windows File ACLs to UNIX POSIX ACLs</title>

        <para>
        Microsoft Windows NT4/200X ACLs must of necessity be mapped to POSIX ACLs.
        The mappings for file permissions are shown in <link linkend="fdsacls">How
        Windows File ACLs Map to UNIX POSIX File ACLs</link>.
        The # character means this flag is set only when the Windows administrator
        sets the <constant>Full Control</constant> flag on the file.
        </para>

        <table frame='all' pgwide='0' id="fdsacls"><title>How Windows File ACLs Map to UNIX POSIX File ACLs</title>
        <tgroup cols='2'>
                <colspec align="left"/>
                <colspec align="center"/>
                <thead>
                <row>
                        <entry align="left">Windows ACE</entry>
                        <entry align="center">File Attribute Flag</entry>
                </row>
                </thead>
                <tbody>
                <row>
                        <entry><para>Full Control</para></entry>
                        <entry><para>#</para></entry>
                </row>
                <row>
                        <entry><para>Traverse Folder/Execute File</para></entry>
                        <entry><para>x</para></entry>
                </row>
                <row>
                        <entry><para>List Folder/Read Data</para></entry>
                        <entry><para>r</para></entry>
                </row>
                <row>
                        <entry><para>Read Attributes</para></entry>
                        <entry><para>r</para></entry>
                </row>
                <row>
                        <entry><para>Read Extended Attribures</para></entry>
                        <entry><para>r</para></entry>
                </row>
                <row>
                        <entry><para>Create Files/Write Data</para></entry>
                        <entry><para>w</para></entry>
                </row>
                <row>
                        <entry><para>Create Folders/Append Data</para></entry>
                        <entry><para>w</para></entry>
                </row>
                <row>
                        <entry><para>Write Attributes</para></entry>
                        <entry><para>w</para></entry>
                </row>
                <row>
                        <entry><para>Write Extended Attributes</para></entry>
                        <entry><para>w</para></entry>
                </row>
                <row>
                        <entry><para>Delete Subfolders and Files</para></entry>
                        <entry><para>w</para></entry>
                </row>
                <row>
                        <entry><para>Delete</para></entry>
                        <entry><para>#</para></entry>
                </row>
                <row>
                        <entry><para>Read Permissions</para></entry>
                        <entry><para>all</para></entry>
                </row>
                <row>
                        <entry><para>Change Permissions</para></entry>
                        <entry><para>#</para></entry>
                </row>
                <row>
                        <entry><para>Take Ownership</para></entry>
                        <entry><para>#</para></entry>
                </row>
                </tbody>
        </tgroup>
        </table>

        <para>
        As can be seen from the mapping table, there is no one-to-one mapping capability, and therefore
        Samba must make a logical mapping that will permit Windows to operate more-or-less the way
        that is intended by the administrator.
        </para>

        <para>
        In general the mapping of UNIX POSIX user/group/other permissions will be mapped to
        Windows ACLs. This has precedence over the creation of POSIX ACLs. POSIX ACLs are necessary
        to establish access controls for users and groups other than the user and group that
        own the file or directory.
        </para>

        <para>
        The UNIX administrator can set any directory permission from within the UNIX environment.
        The Windows administrator is more restricted in that it is not possible from within 
        Windows Explorer to remove read permission for the file owner.
        </para>

        </sect3>

        <sect3>
        <title>Mapping of Windows Directory ACLs to UNIX POSIX ACLs</title>

        <para>
        Interesting things happen in the mapping of UNIX POSIX directory permissions and
        UNIX POSIX ACLs to Windows ACEs (Access Control Entries, the discrete components of
        an ACL) are mapped to Windows directory ACLs.
        </para>

        <para>
        Directory permissions function in much the same way as shown for file permissions, but
        there are some notable exceptions and a few peculiarities that the astute administrator
        will want to take into account in the setting up of directory permissions.
        </para>

        </sect3>

        </sect2>
</sect1>

<sect1>
<title>Common Errors</title>

<para>
File, directory, and share access problems are common topics on the mailing list. The following
are examples recently taken from the mailing list.
</para>


        <sect2>
        <title>Users Cannot Write to a Public Share</title>

        <para>
        The following complaint has frequently been voiced on the Samba mailing list: 
        <quote>
        We are facing some troubles with file/directory permissions. I can log on the domain as admin user (root),
        and there's a public share on which everyone needs to have permission to create/modify files, but only
        root can change the file, no one else can. We need to constantly go to the server to
        <userinput>chgrp -R users *</userinput> and <userinput>chown -R nobody *</userinput> to allow
        other users to change the file.
        </quote>
        </para>

        <para>
        Here is one way the problem can be solved:
        </para>

        <procedure>
                <step>
                        <para>
                        Go to the top of the directory that is shared.
                        </para>
                </step>

                <step>
                        <para>
                        Set the ownership to whatever public user and group you want
<screen>
&prompt;find `directory_name' -type d -exec chown user:group {}\;
&prompt;find `directory_name' -type d -exec chmod 2775 {}\;
&prompt;find `directory_name' -type f -exec chmod 0775 {}\;
&prompt;find `directory_name' -type f -exec chown user:group {}\;
</screen>
                        </para>

                        <note><para>
                        The above will set the <constant>SGID bit</constant> on all directories. Read your
                        UNIX/Linux man page on what that does. This ensures that all files and directories
                        that are created in the directory tree will be owned by the current user and will
                        be owned by the group that owns the directory in which it is created.
                        </para></note>
                </step>
                <step>
                        <para>
                        Directory is <replaceable>/foodbar</replaceable>:
<screen>
&prompt;<userinput>chown jack:engr /foodbar</userinput>
</screen>
                        </para>

                        <note>
                        <para>This is the same as doing:</para>
<screen>
&prompt;<userinput>chown jack /foodbar</userinput>
&prompt;<userinput>chgrp engr /foodbar</userinput>
</screen>
                        </note>
                </step>
                <step>
                        <para>Now type: 

<screen>
&prompt;<userinput>chmod 2775 /foodbar</userinput>
&prompt;<userinput>ls -al /foodbar/..</userinput>
</screen>
                        </para>
                
                        <para>You should see:
<screen>
drwxrwsr-x  2 jack  engr    48 2003-02-04 09:55 foodbar
</screen>
                        </para>
                </step>
                <step>

                <para>Now type:
<screen>
&prompt;<userinput>su - jill</userinput>
&prompt;<userinput>cd /foodbar</userinput>
&prompt;<userinput>touch Afile</userinput>
&prompt;<userinput>ls -al</userinput>
</screen>
                </para>

                <para>
                You should see that the file <filename>Afile</filename> created by Jill will have ownership
                and permissions of Jack, as follows:
<screen>
-rw-r--r--  1 jill  engr     0 2007-01-18 19:41 Afile
</screen>
                </para>
                </step>

                <step>
                <para>
                If the user that must have write permission in the directory is not a member of the group
                <emphasis>engr</emphasis> set in the &smb.conf; entry for the share:
                <smbconfblock>
<smbconfoption name="force group">engr</smbconfoption>
                </smbconfblock>
                </para>
        </step>
        </procedure>
        </sect2>


        <sect2>
                <title>File Operations Done as <emphasis>root</emphasis> with <emphasis>force user</emphasis> Set</title>

                <para>
                When you have a user in <smbconfoption name="admin users"/>, Samba will always do file operations for
                this user as <emphasis>root</emphasis>, even if <smbconfoption name="force user"/> has been set.
                </para>
        </sect2>
        
        <sect2>
                <title>MS Word with Samba Changes Owner of File</title>

                <para>
                <emphasis>Question:</emphasis> <quote>When user B saves a word document that is owned by user A,
                the updated file is now owned by user B.  Why is Samba doing this? How do I fix this?</quote>
                </para>

                <para>
                <emphasis>Answer:</emphasis> Word does the following when you modify/change a Word document: MS Word creates a new document with
                a temporary name. Word then closes the old document and deletes it, then renames the new document to the original document name.
                There is no mechanism by which Samba can in any way know that the new document really should be owned by the owners
                of the original file. Samba has no way of knowing that the file will be renamed by MS Word. As far as Samba is able
                to tell, the file that gets created is a new file, not one that the application (Word) is updating.
                </para>

                <para>
                There is a workaround to solve the permissions problem. It involves understanding how you can manage file
                system behavior from within the &smb.conf; file, as well as understanding how UNIX file systems work. Set on the directory
                in which you are changing Word documents: <command>chmod g+s `directory_name'.</command> This ensures that all files will
                be created with the group that owns the directory. In &smb.conf; share declaration section set:
                </para>

                <para>
                <smbconfblock>
                <smbconfoption name="force create mode">0660</smbconfoption>
                <smbconfoption name="force directory mode">0770</smbconfoption>
                </smbconfblock>
                </para>

                <para>
                These two settings will ensure that all directories and files that get created in the share will be readable/writable by the
                owner and group set on the directory itself.
                </para>
                
        </sect2>

</sect1>

</chapter>